Improved homogenization of fly's eye condenser setups under coherent illumination using chirped microlens arrays

Abstract

Fly's eye condensers are commonly used for the beam shaping of an arbitrary input intensity distribution into a top hat. The setup usually consists of a Fourier lens and two identical regular microlens arrays - often referred to as tandem lens array - where the second one is placed in the focal plane of the first microlenses. Due to the periodic structure of the regular arrays the output intensity distribution is modulated by equidistantly located sharp intensity peaks. In a chirped array, the inflexibility of a regular structure has been overcome. Hence, an array can be formed which is non-periodic and consequently the equidistantly located intensity peaks can be suppressed. A far field speckle pattern results with more densely and irregularly located intensity peaks leading to an improved homogeneity of the intensity distribution. In contrast to stochastic arrays, chirped arrays consist of individually shaped lenses defined by a parametric description of the cells optical function which can be derived completely from analytical functions. This gives the opportunity to build up tandem array setups enabling to achieve far field intensity distribution with an envelope of a top hat. We propose a new concept of a fly's eye condenser incorporating a chirped tandem microlens array for the generation of a top hat far field intensity distribution with improved homogenization under coherent illumination. Considerations for the design of the irregular micro lens arrays and measurements of far field intensity distributions obtained from first prototypes generated by reflow of photoresist are presented.